Contralateral efferent neurons can be detected in the hindbrain outside of rhombomere 4.

A group of efferent neurons whose bodies are located contralaterally and extend projections across the ventral midline of the hindbrain is considered as a rhombomere 4-specific characteristic. These neurons contribute to the vestibulo-acoustic nerve. At the level of rhombomere 2, a similar kind of efferents have only been described as a result of several experimental manipulations and have been interpreted as being due to rhombomere 2 acquiring rhombomere 4 identity. Here is shown that contralateral efferents can also be detected in rhombomere 2 of normal mouse and chicken hindbrains. These findings indicate that neural processes crossing the midline should not be considered as a rhombomere 4-specific characteristic. They also imply that the formation of the contralateral efferents at different rostro-caudal levels might be under different genetic controls, because Hoxb-1, which is not expressed in rhombomere 2, seems to be essential for their proper formation in rhombomere 4.